Calcium phosphates are the main minerals in human bone, enamel, atherosclerosis, and dental calculus. Amorphous precursors may play a key role in biomineralization. We studied the formation and transformation of calcium phosphate particles of amorphous phase by stopped-flow spectrophotometry, simultaneous measurements of particle size and solution pH, and high-resolution transmission electron microscopy. Ion pairs and clusters formed in the first few seconds. They then constituted initial amorphous phase containing protonated phosphates and hydrated calcium ions, which was different from that containing Ca(9)(PO(4))(6). Crystalline domains developed at multiple sites inside the primary particles of the amorphous phase. With the consuming of interdomain constituents, these particles partially collapsed, liberating crystallites and inducing rapid precipitation. This study sheds new light on the understanding of crystallization in amorphous phase, as well as the induction period in precipitation kinetics.